Proposals have been made to fit aircraft landing gear with independent drive devices including electric motors located as close as possible to wheels carried by the undercarriages in order to drive the wheels in rotation and thus enable the aircraft to move without help from the engines of the aircraft.
It is tempting to fit the wheels with hydraulic motors suitable for use both for driving movement of the aircraft and also for braking it, rather like the brakes on construction machines fitted with such motor-driven wheels, and making use of hydrostatic transmission.
Nevertheless, the use of such wheels assumes that a source of hydraulic pressure is available. On an aircraft, the pumps that generate hydraulic pressure are driven by the engines of the aircraft, which means that those engines must therefore be active when the aircraft is moving, even if they are idling. However, it is known that the residual thrust from engines that are idling is itself sufficient to cause the aircraft to advance, which means that it is necessary constantly to apply the brakes to the wheels in order to prevent the aircraft from moving.
Proposals have been made, in document WO 2010/0046520 to form an auxiliary hydraulic circuit in which a pump is actuated by an auxiliary power unit (APU) of the aircraft. The pump may be driven mechanically by a shaft of the APU or it may have an electric motor that takes its electrical power from the power delivered by an alternator that is driven by the APU. Thus, the independent drive devices may be powered hydraulically by means of an auxiliary hydraulic circuit pressurized by an auxiliary pump activated by means of the APU, thereby enabling the aircraft to be moved without help from its engines.
Nevertheless, the friction brakes of the aircraft are also hydraulically activated. It is known that the brakes are powered by the main hydraulic circuits of the aircraft. Certain aircraft are fitted with an alternative hydraulic circuit, that take over from the main circuits when they have failed or are not activated. The alternative braking circuit includes an accumulator that enables some limited number of braking operations to be performed when the alternative braking circuit is itself not being pressurized, in particular because the engines are stopped. Nevertheless, the number of braking operations that it is possible to perform is very limited. An intermediate solution for increasing the number of possible braking operations would be to increase the size of the accumulator, but that quickly becomes penalizing in terms of weight.
Document U.S. Pat. No. 3,807,664 describes an aircraft fitted with a braking circuit and a circuit for powering independent drive means.
Those two circuits are fed with pressure either by a pump of the aircraft driven by its engines, or by an auxiliary pump driven by the APU.